1. Field of the Invention
The present invention relates to a scanning optical device and a multi-beam scanning optical device and, more particularly, to a device suitably used for a laser beam printer, a digital copying machine, and the like, in which a light beam emitted from a light source means is guided onto a surface to be scanned as a recording medium surface through a light deflector such as a rotating polygonal mirror, and the light beam is scanned on the scanned surface, thereby recording information such as characters.
2. Related Background Art
In a conventional scanning optical device used for laser beam printer, a digital copying machine, and the like, a light beam emitted from a light source means is deflected by a deflection means, and the deflected light beam forms a spot on a photosensitive drum surface as a scanned surface via a scanning optical means, thereby scanning the light beam on the scanned surface.
Recently, to meet demands for reductions in cost and size, a scanning optical means using a plastic lens made of a plastic material has been widely used as a scanning optical means in a scanning optical device of this type. However, the refractive index of the plastic lens changes with changes in a use environment (temperature change, in particular). For this reason, in a scanning optical device using a plastic lens, focus changes, magnification changes, and the like occur in the main scanning direction due to environmental variations.
To solve this problem, in the scanning optical device disclosed in Japanese Patent Application Laid-Open No. 10-68903, a diffraction optical element is formed on the lens surface to correct focus changes, magnification changes, and the like in the main scanning direction with changes in the temperature of the scanning optical device by using power changes in the refraction and diffraction units of the scanning optical means and wavelength variations in the semiconductor laser as a light source means.
FIG. 1 is a sectional view (main scanning cross-section) of the main part of the scanning optical device in the above reference in the main scanning direction. Referring to FIG. 1, a light beam emitted from a light source means 11 constituted by a semiconductor laser and the like is converted into a substantially collimated light beam by a collimator lens 12. This substantially collimated light beam is shaped into an optimal beam shape by an aperture stop 13 and strikes a cylindrical lens 14. The cylindrical lens 14 has a power in the sub-scanning direction and forms a light beam image elongated in the main scanning direction near a deflection surface 15a of the light deflector 15 constituted by a rotating polygonal mirror and the like. In this case, the main scanning direction is a direction perpendicular to the deflection scanning direction, and the sub-scanning direction is a direction perpendicular to the deflection scanning direction. This applies to the following description. The light beam is reflected/deflected by the light deflector 15 at an equal angular velocity to form a spot on a photosensitive drum surface (recording medium surface) 17 as a scanned surface via an f.theta. lens 16 which is a single element lens serving as a scanning optical means having an f.theta. characteristic. This light beam is scanned on the photosensitive drum surface 17 at an equal velocity.
According to this reference, a diffraction optical element 18 is formed on that surface of the f.theta. lens 16 which is located on the scanned surface 17 side to correct focus changes, magnification changes, and the like in the main scanning direction, which may occur with variations in the temperature of the scanning optical device, by using power changes in the refraction and diffraction units of the scanning optical means 16 and wavelength variations in the semiconductor laser 11 as the light source means.
According to this reference, since a single element lens is used for the scanning optical means, the degree of freedom in aberration correction is low. This tends to pose difficulty in meeting the requirement for an increase in resolution. FIG. 2 shows the comatic aberration of a full-pupil light beam in the main scanning direction in the scanning optical means according to the first embodiment in the reference as an example of this case. Referring to FIG. 2, the abscissa represents the image height (unit: mm); and the ordinate, the comatic aberration (unit: wavelength .lambda.). In this case, as comatic aberration, the value obtained by dividing the difference between .+-. full-pupil wavefront aberrations in the main scanning cross-section by 2 is used, and its asymmetrical component is evaluated. In this case, the spot diameter in the main scanning direction is set to 80 .mu.m. As is obvious from FIG. 2, a comatic aberration of about 0.12 .lambda. occurs at the middle image height.
In general, when a comatic aberration of 0.1 .lambda. or more occurs, the formation of side lobe becomes conspicuous in the main scanning direction. As a result, an appropriate spot shape cannot be obtained, affecting the printed image. If the spot diameter is reduced to increase the resolution, the comatic aberration abruptly increases to disturb the spot shape.
In addition, when the spot diameter is further reduced in the sub-scanning direction as well to increase the resolution, the occurrence of wavefront aberration becomes noticeable, disturbing the spot shape. In general, to correct tilt in a scanning optical means, a light beam is temporarily imaged in the sub-scanning direction near the deflection surface of a deflection means to set the deflection surface and the scanned surface conjugate with each other. For this reason, since the power in the sub-scanning direction is larger than that in the main scanning direction, large focus changes occur with environmental variations. When the spot shape is reduced to increase the resolution, the depth of focus decreases. As a consequence, the spot diameter in the sub-scanning direction greatly changes with a focus change.
Recently, to meet the demand for an increase in speed, various multi-beam scanning optical devices for scanning a plurality of light beams on a scanned surface have been proposed.
When, however, the respective light beams differ in wavelength, since the f.theta. characteristic varies with wavelength, chromatic aberration of magnification occurs. For this reason, the scanning length on the scanned surface varies depending on the respective light beams. Even if, therefore, the start positions of the respective light beams in write operation are aligned with each other, the end positions in the write operation differ from each other, causing jitter in the image formed.
In consideration of processability a diffraction optical element is preferably formed on a flat surface. In this case, however, since the degree of freedom in aberration correction decreases, it is difficult to obtain good optical performance. This arrangement greatly influences the partial magnification or f.theta. characteristics which can be properly corrected by using an aspherical shape. As a consequence, for example, the number of lenses increases, and the lens size increases.